This discussion really ought to start with a bit about
semiconductors
as materials.

Semiconductors
are crystals that, in their pure state, are resistive (that
is, their electrical properties lie between those of
conductors
and insulators) -- but when the proper impurities
are added (this process is called doping)
in trace amounts (often measured in parts per billion),
display interesting and useful properties.

A bit of historyThe oldest ancestor of semiconductor devices was the
crystal detector, used in
early wireless radios. This device (patented by a German
scientist, Ferdinand Braun, in 1899) was made of a single
metal wire (fondly called a "cat's whisker") touching
against a semiconductor
crystal. The result was a "rectifyingdiode" (so
called because it has two terminals), which lets current
through easily one way, but hinders flow the other way. By
1930, though, vacuum-tube diodes
had all but replaced the smaller but much quirkier crystal
detector. The crystal and "cat's whisker" were left to
languish as a kids' toy in the form of "crystal radios."

The development of radar during World War II did much to
revive the fortunes of crystal detectors (and, as a result,
that of semiconductors)
-- although temperamental, crystals were better than
vacuum-tube diodes
at rectifying
the high frequencies used by radar. So, during the war, much
effort was put into improving the semiconductors,
mostly silicon and germanium, used in crystal detectors. At
about the same time, Russell Ohl
at Bell Laboratories discovered that these materials could
be "doped"
with small amounts of foreign atoms to create interesting
new properties.

Depending on the selection of impurities (often called
dopants) added, semiconductor
material of two electricallly-different types can be created
-- one that is electron-rich (called N-type,
where N stands for Negative), or one that is
electron-poor (called P-type,
where P stands for Positive). Most of the "magic" of
semiconductor devices occurs at the boundary between
P-type
and N-typesemiconductor
material -- such a boundary is called a P-N
junction. Ohl and his colleagues found that such a
P-N
junction made an effective diode
(but more on that later).

For BEAM devices, we'll be concerned with two broad types
of semiconductor devices: